Lot's of good advice here. I cook these regularly for events, using a Yoder pellet (big 1500 or the little 640). Even on a controlled cooker like a pellet, brisket are really challenging because you have a thick fatty cap and that thin, relatively lean flat. I looked at the Mixon recipe and I think the cooking method is pretty sound, but I would hate that thing because of all the rubs and injections. Brisket, IMHO, needs plenty of salt and plenty of black pepper, with a little ground red pepper if you want a bit of heat, nothing else. Injections and rubs make that thing taste like some kind of baloney or potted meat product, yuk.
I'd try the suggested method of opening the vent on the egg for at least the first hours of the cook to get a bit more smoke. I like to cook these at 275-295 in an open aluminum pan and shoot to get them to about 195-198 for the flat, then separate the flat and cap (just follow the fat line), wrap the flat super, super tight in foil and let sit, meanwhile dice the cap into about 3/4 inch squares and throw them in a skillet on the grill at about 350 to get some color and finish cooking for about 20 minutes, then glaze them with a sauce. You do it right those squares are like little tender beef marshmallows. make sure you slice the flat against the grain after you've let it cool significantly, if you slice it hot it will dry out.